The present invention relates to an automatic chemical analyzer of direct measurement type, designed to subject a plurality of reaction cells, each containing a sample and a reagent, to photometric measurement to determine the absorbancy of the sample, thus analyzing the sample contained in the cell. More particularly, it relates to the technique of compensating for the measurement values for the difference among the light path lengths of the cells.
An automatic chemical analyzer uses a number of reaction cells shaped like a test tube. Each cell is filled with a sample (e.g., urine or serum) and a reagent reacting with a specific component of the sample. The cells are intermittently moved one after another to a position where the photometric measurement is carried out. Light is applied to each cell and passes through it. The light from the cell is separated by a diffraction grating into spectral components. The attenuation of the light component having a specific wavelength is measured, thus determining the content of a particular component of the sample.
To measure accurately the attenuation of the light caused by the samples, the difference in the light path lengths of the cells must be minimized. More specifically, the reaction cells must have substantially the same inner diameter. However, it is very difficult and it costs very much to manufacture such reaction cells. If provided with such cells, the analyzer will be greatly expensive. Therefore, the analyzers commercially available at present are designed with allowance for the difference ranging from .+-.0.17% to 0.33%.
As Lambert-Beer's law teaches, the difference in the light path length directly affects the accuracy of measurement. The difference ranging from .+-.0.17% to 0.33% cannot, therefore, be neglected, and it is desired that the influence of this difference on the measurement accuracy be minimized.